Si Nanospheres Research Articles

Second harmonic surface response of a composite

Abstract We calculate the nonlinear dipole and quadrupole moments induced at the second harmonic (SH) frequency 2 ω in a dielectric nanosphere by an inhomogeneous monochromatic electric field of frequency ω . We neglect finite size effects and assume that the surface region of the nanosphere is thin enough so that the surface may be considered locally flat. Then, we calculate the nonlinear optical response of a centrosymmetric semiinfinite composite made by the nanospheres. Within the dipole approximation, SH is forbidden in the centrosymmetric bulk, but allowed at the surface of the composite where the symmetry is broken. Therefore, the components of the surface susceptibility tensor, χ ijk , different from zero are calculated. As an application, we evaluate χ ijk for a composite made of Si nanospheres.

Quantum Confinement of Si Nanosphere with Radius Smaller than1.2 nm

We calculate the density of states, the squared optical matrix elementalong the x direction, and the band gap of Si nanosphere withradius r smaller than 1.2 nm using the method of linearcombination of atomic orbitals. It is shown that the quantumconfinement effect of Si nanocrystals exists obviously, fulfilling E = 1.92+0.23/r1.8, but the band gap exhibitsfluctuation with nanocrystal size, which is caused by the danglingbonds of atoms in outer layers. The obtained result indicates that thesurface chemical bonds have larger influence on the energy bandstructure of Si nanosphere when its radius is smaller than 1.2 nm.

A boundary constraint energy balance criterion for small volume deformation

A concept for the deformation resistance of small volumes under contact subjected to displacements in the 2–100 nm regime is proposed. In terms of an energy balance criterion, the external work minus stored elastic energy is consumed by surface energy and plastic energy absorption. The surface energy is interpreted in terms of new area of contact or new area created by slip step emergence or oxide film fracture. The plastic energy absorption is interpreted in terms of dislocation work. As such, this is analogous to a contact mechanics Griffith criterion for the hardness or flow strength of crystalline metals, semiconductors or oxides. Derivations of supporting loads and stresses provided for nanoindenter tips, nanospheres, nanocubes and thin-walled nanoboxes are presented. For comparison, measured hardness and flow strengths of Al, Au, Fe–3%Si, Ti, W and Si nanostructures are reported in the 2–100 nm length scale regime. It is proposed that Si and Ti nanospheres experience a minimum in contact resistance as dictated by a balance in surface energy dominance at small contact and dislocation hardening dominance at large contact. The variation of a strength controlling coefficient, α, with material suggests a length scale dependence on shear and/or pressure activation volumes.

Growth, structure and properties of chains of crystalline-Si nanospheres

We have fabricated novel self-organized nanostructures, chains of crystalline-Si nanospheres via an extension of the vapor–liquid–solid process from metal catalysts. The growth mechanism was revealed through transmission electron microscopy observations and computer simulations. By controlling metal impurities in the catalysts, we have grown dense chains of crystalline-Si nanospheres. We performed Raman scattering measurements of the chains and found that phonons were confined in the Si nanospheres in the chains and that the Si nanospheres were under compressive stress.

One-phonon Raman scattering studies of chains of crystalline-Si nanospheres

Chains of crystalline-Si nanospheres were studied by means of Raman scattering spectroscopy. We found that the one-phonon Raman scattering peak from the chains was asymmetric and broader than that from bulk Si. This phenomenon can be attributed to a phonon confinement in the silicon nanospheres. The phonon confinement became more obvious by decreasing the size of the silicon nanospheres in the chains. We also found that the Si nanospheres in the chains were under compressive stress by the covering oxide layers through the analysis of the Raman shift.

Bulk-quantity Si nanosphere chains prepared from semi-infinite length Si nanowires

Bulk-quantity Si nanosphere chains have been fabricated. This is accomplished via the spheroidization of Si nanowires of semi-infinite lengths. The process has been extensively investigated by transmission electron microscopy. The nanosphere chains consisted of equally spaced Si crystalline nanospheres connected by Si-oxide bars. The transition from Si nanowires to Si nanosphere chains was determined by the annealing temperature, ambient pressure, initial Si nanowire diameters, and the oxide state of the outer layers of Si nanowires. The relationships between the geometry (size and spacing) of Si nanospheres, the initial state (diameter and oxide state) of Si nanowires, and the experimental conditions are discussed.